1. Field of the Invention
This invention relates to a polyester polymerization catalyst, polyester produced by using the same and a process for producing polyester, and in particular to a novel polymerization catalyst not using a germanium or antimony compound as a major catalytic component, polyester produced by using the same and a process for producing polyester.
2. Description of the Related Art
Polyesters represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN) etc. are excellent in mechanical and chemical characteristics, and are used in various fields for example in fibers for clothing and industrial materials, films for such as packaging or magnetic tapes, sheets, hollow molded articles such as bottles, casings for electrical or electronic parts, and other molded articles of engineering plastics, depending on the characteristics of each polyester.
As typical polyester, polyester comprising an aromatic dicarboxylic acid and an alkylene glycol as major constituent components, for example polyethylene terephthalate (PET), is industrially produced by esterification or transesterification of terephthalic acid or dimethyl terephthalate and ethylene glycol to produce bis(2-hydroxyethyl) terephthalate which is then subjected to polycondensation at high temperatures in vacuo in the presence of a catalyst.
As a conventional polyester polymerization catalyst used in polycondensation of polyester, antimony trioxide has been used widely. Antimony trioxide is an inexpensive and highly active catalyst, but when antimony trioxide is used as a major component, that is, when it is used in such an amount as to exhibit a practical rate of polymerization, an antimony metal is precipitated thus causing problems such as gray disdiscoloration or generation of insoluble particles in polyester. For this reason, polyester absolutely free of antimony or not containing antimony as a major catalytic component is desired.
The above-described insoluble particles in polyester causes the following problems:
In polyester for film, the antimony metal precipitated serves as insoluble particles in polyester, which causes not only deposits around die during melt extrusion but also deficiency in the surface of film. Further, when the polyester with insoluble particles is used as a starting material of hollow molded articles, it is difficult to obtain hollow molded articles excellent in transparency.
The insoluble particles in polyester for fibers serves as insoluble particles not only causing a reduction in the strength of fibers, but also deposits around spinnerets during spinning. In production of polyester fibers, a polyester polymerization catalyst not causing formation of insoluble particles is desired from the viewpoint of productivity.
As a method of solving the problem described above, an attempt had been made at preventing gray discoloration and formation of insoluble particles in PET while using antimony trioxide as a catalyst. In Japanese Patent No. 2666502, for example, formation of black insoluble particles in PET is prevented by using antimony trioxide, a bismuth compound and a selenium compound as a polycondensation catalyst. Further, JP-A 9-291141 describes that precipitation of an antimony metal is prevented when antimony trioxide containing sodium and iron oxides is used as a polymerization catalyst. However, these polycondensation catalysts cannot achieve the object of reducing the content of antimony in polyester.
As a method of solving the problem of the antimony catalyst in uses requiring transparency of PET bottles etc., for example JP-A 6-279579 discloses a method of improving transparency by prescribing the proportion of antimony and phosphorus compounds used. However, it cannot be said that hollow molded articles made of polyester obtained by this method are sufficiently transparent.
Further, JP-A 10-36495 discloses a process for producing polyester excellent in transparency, which comprises use of antimony trioxide, phosphoric acid and a sulfonic acid compound. However, polyester obtained by such a method has lower thermal stability, and there is the problem of a high content of acetaldehyde in the resultant hollow molded article.
Polycondensation catalysts substituted for antimony containing catalysts such as antimony trioxide have also been examined, and titanium compounds such as tetraalkoxy titanate or tin compounds have previously been proposed, but have a problem that polyester produced by using these compounds is easily thermally degraded during melt molding, and the polyester is significantly discolored.
In an attempt at solving the problem arising when such titanium compounds are used as the polycondensation catalyst, for example JP-A 55-116722 proposes a method of simultaneously using tetraalkoxy titanate in combination with a cobalt salt and a calcium salt. Further, JP-A 8-73581 proposes a method of using tetraalkoxy titanate in combination with a cobalt compound as the polycondensation catalyst and simultaneously using a optical brightener. By these techniques, PET discoloration occurring when tetraalkoxy titanate is used as the polycondensation catalyst can be reduced, but prevention of thermal degradation of PET cannot be efficiently achieved.
In another attempt at preventing thermal degradation during melt molding of polyester polymerized in the presence of a titanium compound as the catalyst, for example JP-A 10-259296 describes a method of adding a phosphorus compound after polymerization of polyester in the presence of the titanium compound as the catalyst. However, effective mixing of the additive with the polymer after polymerization is technically difficult and leads to higher costs, so this prior art method is not practically used under the present circumstances. It is known that aluminum compounds are generally inferior in catalytic activity. Among the aluminum compounds, aluminum chelate compounds are reported to have a higher catalytic activity as the polycondensation catalyst than other aluminum compounds, but the aluminum chelate compounds cannot be said to have a sufficient catalytic activity as compared with the antimony compound or titanium compound described above, and polyester polymerized for a long time using the aluminum compound as the catalyst has the problem of lower thermal stability.
A method of adding an alkali metal compound to an aluminum compound to form a polyester polymerization catalyst having a sufficient catalytic activity is also known. When such a known catalyst is used, polyester excellent in thermal stability can be obtained, but this catalyst using an alkali metal compound in combination should be added in a larger amount in order to attain a practical catalytic activity, and as a result, there arises at least one of the following problems attributable to the alkali metal compound in the resultant polyester polymer.
The amount of insoluble particles is increased so that when the polyester is used in fibers, the spinnability and the physical properties of fibers are getting worse, and when used in films, the physical properties of films are getting worse.
The hydrolytic stability of the resultant polyester polymer is lowered, and due to formation of insoluble particles, the transparency thereof is lowered.
There arises the phenomenon of deficiency in color tone of the resultant polyester polymer, that is, yellow discoloration of the polymer, and when the polyester is used in films or hollow bottles, there arises the problem of degradation in the color tone of the molded articles.
During production of molded articles by melt molding, filter pressure is increased due to clogging of a filter with insoluble particles, to lower productivity.
As an non-antimony catalyst for giving polyester free of the problem described above, a germanium compound has been practically used, but this catalyst has a problem that it is very expensive and easily distilled away from a reaction system during polymerization, thus changing the concentration of the catalyst in the reaction system and making control of polymerization difficult, so use of the germanium component as a major catalytic component is problematic.
For preventing thermal degradation of polyester during melt molding, there is also a method of removing a catalyst from polyester. JP-A 10-251394 discloses a method of removing a catalyst from polyester wherein a polyester resin is brought into contact with an extractant as supercritical fluid in the presence of an acidic substance. However, the method of using such supercritical fluid is technically difficult and leads to higher costs for products, and is thus not preferable.
For the reasons described above, there is demand for a polymerization catalyst which comprises as a major catalytic component a metal component other than antimony and germanium, is excellent in catalytic activity and gives polyester free of the problems described above.
This invention provides a novel polyester polymerization catalyst comprising, as a major catalytic metal component, a metal component other than antimony or germanium, polyester produced by using the same and a process for producing polyester by using the catalyst.
Further, this invention provides a novel polyester polymerization catalyst comprising not an antimony compound or a germanium compound as a major catalytic component but aluminum as a major metal component, polyester produced by using the same and a process for producing polyester by using the catalyst.
Another object of this invention is to provide a polyester polymerization catalyst which comprises not an antimony compound or a germanium compound as a major catalytic component but aluminum as a major metal component, is excellent in catalytic activity, and without deactivating or removing the catalyst, gives polyester not only excellent in thermal stability by effectively inhibiting thermal degradation during melt molding but also superior in color tone and transparency without formation of insoluble particles.
Further, this invention provides polyester which improves thermal stability, the problem of formation of insoluble particles and productivity by using the catalyst during melt molding thereof into films, hollow molded articles such as bottles, fibers and engineering plastics, and gives products superior in quality level by using virgin resin thereof or by reutilizing scraps thereof generated during molding, as well as a process for producing polyester by using the polyester polymerization catalyst.
Another object of this invention is to provide a polyester polymerization catalyst giving polyester excellent in thermal oxidation stability and hydrolytic stability, polyester excellent in thermal oxidation stability and hydrolytic stability, and a process for producing polyester by using the polyester polymerization catalyst.
Another object of this invention is to provide molded articles such as fibers, films and bottles comprising polyester produced by using the catalyst. As a result of extensive study for solving the problems described above, the present inventors found that an aluminum compound is originally inferior in catalytic activity, but the aluminum compound when used in the coexistence of a phosphorus compound having a specific structure can have a sufficient activity as the polymerization catalyst, thus arriving at the present invention. When the polycondensation catalyst of this invention is used, polyester excellent in qualities which does not use an antimony compound or a germanium compound can be obtained.
That is, this invention provides a polyester polymerization catalyst comprising an aluminum compound and a phosphorus compound having a specific structure, polyester produced by using the same and a process for producing polyester, in order to solve the problems described above.